The art of sail-making has gone from sewing together sheets of canvas to panels of polymeric materials. Many modern sails are now made from exotic materials having a sandwich structure where the outer layers of the sandwich usually are made from some polymer material and sandwiched between the materials are a pattern of yarns. By arranging the yarns in specific patterns and selecting yarns made from strong materials, it is possible to guide the stress to which the sail is exposed in use to the claw, tack and head of the sail, such that by arranging the yarns in an optimum manner in the sail matrix, it is possible to manufacture very strong sails which at the same time are able to attain the desired curvature.
Typically, the material layers are made from polyester polyvinylidene halides polyurethanes or similar material, whereas the yarn is usually selected among aramids, carbon fibres, Mylar fibres, polyester or Dynema fibres. All these fibres are very strong, but as with all the materials mentioned previously they have a tendency to shrink and if they are made from different materials, the shrinkage is also different from material to material.
In the art, a method of manufacturing such a laminate material is suggested in U.S. Pat. No. 5,355,820 wherein a membrane table having a plenum provided with openings such that a vacuum may be established substantially across the entire table's plenum is provided.
In connection with the present invention it should be noted that the term vacuum is not to be construed as the scientific term vacuum, but merely covers the term under-pressure and sufficient under-pressure such that if a laminate or membrane is placed on the plenum of the vacuum table, the vacuum openings and the under-pressure supplied to the vacuum openings will be able to substantially fix the laminate relative to the plenum.
Along the longitudinal edges of the plenum, rails are provided such that a gantry spanning from one side of the plenum to the other may freely travel along the plenum of the table. The gantry is provided with a device for distributing the yarn in patterns across a first laminate fixated on the plenum via the vacuum openings as explained above. In order to maintain the yarns at a certain tension pegs or pins are provided such that as the yarn is made to change directions, this will occur around one or more pins/pegs. In this manner and by providing the yarns with an adhesive it is possible to provide the yarns in a specific pattern and with tension onto a first membrane. After having distributed the yarns in the desired pattern in order to provide the membrane material with the desired strength characteristics, an upper membrane material is provided on top of the yarns. After having placed the upper material, the laminate consisting of a lower material layer, the yarns and the upper material layer is exposed to pressure, for example supplied by a roller or by a vacuum bagging process. The roller is simply a large roller which is passed over the laminate while being heated such that the lamination process of the membrane is carried out by means of the roller and the heat.
In the vacuum bagging process a large foil is placed and sealed on top of the laminate, and a vacuum is applied to this foil which will then suck on to the laminate and thereby exert a certain pressure on the laminate. The process of removing air/gas trapped between the laminates or having the solvents used with the adhesive diffuse, is, however, difficult and time-consuming at best.
Typically, the laminates manufactured according to this process are later cut out in pre-designed panels which are assembled into the finished sail such that each panel will have desired properties for a specific position in the sail. In this manner it is possible to arrange the yarns in each panel such that the optimum force distribution will be provided without the sails having excessive strength and thereby excessive weight which is detrimental to sail handling and sailing characteristics overall. Also for cost considerations it is interesting only to use expensive exotic materials only where they are needed.
In an attempt to avoid using panels which need to be sewn together U.S. Pat. No. 5,097,784 proposes to create the sail in a single piece by distributing the laminates and the yarns on a mould having a convex or concave shape corresponding to the desired shape of the sail in the optimum use situation. In this process, however, it is very difficult to laminate the two layers sandwiching the yarns together in a reliable and sufficiently strong manner, but one advantage is that uninterrupted yarns and thereby optimum force distribution is achievable, but the laminate as such is relatively weak and very prone to manufacturing faults. Furthermore, due to the heat and pressure treatment during lamination, differential shrinkage, in the different materials will occur.
Common for the two processes mentioned above is the fact that they use a combination of heat and pressure in order to laminate the two materials together sandwiching the yarns. By heating the laminates, these will shrink whereby the dimensions of the finished membrane will be changed. Furthermore, if vacuum technique is used to fixate the laminate sheet in contact with the plenum or mould, this material layer will be less prone to shrink in that it is fixated by the vacuum openings in comparison to the upper layer which is simply laid out and in some prior art methods fixated by means of clamps or pegs along the edges of the sail in order to try to keep the shape stability as high as possible.
Furthermore, by applying pressure by means of rollers which is suggested for both the above-mentioned prior art methods the bulk of the pressure exerted by the rollers will be exerted onto the laminates where the yarns are present in that the material thickness in these places will be substantially thicker than the material thickness where no yarn is present such that the roller is very unlikely to ever effectively press/squeeze together the sheet materials as such, This of course potentially causes the finished sandwich material to have a weakened structure.
In EP 0670778 is also described a method of manufacturing a laminate sail where the problems relating to shrinkage, deformation etc. are addressed by providing the reinforcement filaments as extremely thin filaments such that the difference in thickness, especially between where two filaments cross each other and in positions where only the outer layers are present is reduced to a minimum such that the laminate may be created by an autoclaving process, heat and pressure or simply bonding.
This process, however, requires that the reinforcement yarns have a thickness of approximately 5 microns which requires special filaments, which are not commercially available. A further problem with these types of filaments is the fact that they may not be as well-proven both with respect to strength characteristics and with respect to long-time wear characteristics as the well-known and well-proven filaments/yarns.